In a typical power distribution system, power is distributed over power transmission lines between a power generator and a load. The efficiency of such a distribution system improves when it is operated near unity power factor. The transmission of power, however, almost inevitably involves current induced magnetic fields, which leads to a lagging power factor of the distribution system. In addition, since loads vary at different times of the day, week and seasons, this power factor is constantly changing.
In order to improve the efficiency of the distribution system, the power factor is often corrected by introducing a source of leading power factor to the system, such as capacitance. Consequently, capacitors are usually shunt-connected across the power transmission lines and can be either energized continuously or switched on and off during changing load cycles. Moreover, capacitors can either be grounded or ungrounded. In most cases, capacitors are automatically discharged when switched out of the system.
Switching of capacitors into a power system, however, may cause voltage transients, especially when capacitors are switched into high voltage systems. For example, in the case of a grounded capacitor, one side of the capacitor is usually connected to a substation ground grid. Switching such a capacitor into a distribution system may cause the current inrush through the capacitor since it may be at a different potential than the power line it is being connected to. This causes voltage transients in the ground grid of the substation, as well as voltage transients in the power line that degrade the quality of the power supplied to the customer. These current transients may also blow the protective fuses which are connected in series with the capacitor, decrease the lifespan of the capacitor, damage substation control circuitry, and cause interference with nearby electrical controls.
In cases of multiple phase transmission systems, these problems are multiplied. For example, in a three phase transmission system, three capacitors may have to be switched into the system, and, as each capacitor is switched into the system, it may cause transients. Accordingly, multiple phase distribution systems present additional complexities when switching capacitors into the system.
It is known that to decrease these transients, each grounded capacitor should be switched into its respective line when the voltage on that line crosses zero potential. This way, the capacitor and the line are at the same potential. This decreases current surges and hence decreases voltage transients.
More particularly, for a typical three-phase transmission system, each phase is transmitted 120 degrees apart. This produces voltage zero crosses that are 60 degrees apart. Thus, to switch three grounded capacitors into this system, the first capacitor for the first phase is switched in when the voltage signal carried in the first phase crosses zero potential, followed by the second capacitor for the second phase being switched in 60 degrees later, followed by the third capacitor being switched in 60 degrees after the second.
In practice, however, it is difficult to switch a capacitor into a line exactly when the voltage signal in the line crosses zero potential. This is primarily due to arcing between the contacts of the capacitor switch as it closes, which is commonly referred to as pre-striking. Pre-striking causes current to flow between the contacts of the capacitor switch before they actually close, which means that the capacitor may be inserted into the line before the voltage signal in the line actually crosses zero potential.
To address this problem, it is known to close the capacitor switch shortly after the voltage signal crosses zero potential. This provides a window of time during which the switch can pre-strike. If pre-striking occurs as the contacts move towards each other, it will occur closer to when the voltage signal crosses zero potential. This will decrease the magnitude of any associated voltage transients.
In fact, to address the pre-striking problem, many manufacturers intentionally design their systems such that the capacitor switches close about 1 millisecond after their respective voltage signal crosses zero potential. Thus for a typical 60 hertz system, at 1 millisecond after the zero voltage zero, the voltage signal is 19% of its peak voltage. The capacitor switch is then designed to withstand this voltage, since, in some instances, the switch may not pre-strike and therefore close at 19% of peak voltage. In addition, in situations where the switch pre-strikes, it is unlikely that the switch will pre-strike more than 1 millisecond prior to zero crossing due to the distance between the switch's contacts at that time. Closing the contacts after voltage zero also allows for minor variations in the closing times due to the mechanical nature of the switches.
However, having the capacitor switch close after zero crossing does not completely solve the transient and pre-striking problem. Additional problems arise because the ground grid to which a capacitor is grounded has inductance. Thus, as the capacitor switch closes and pre-strikes, the current that begins to flow through the capacitor and switch can be at very high frequency, for example, around 10,000 hertz. Although this current may not be of a very high magnitude, it can cause a large voltage rise in the ground grid due to the grid's inductance and the high frequency. This, in turn, causes a large voltage rise across the contacts of the remaining open capacitor switches, which makes it more likely that those switches will pre-strike, which, in turn, compounds the voltage transient problem. In other words, if the first switch pre-strikes, the next two switches are more likely to pre-strike, and may, in fact, pre-strike at the same time as the first switch. The pre-striking of these switches adds further transients and further increases the voltage rise across the ground grid. Thus, voltage transients are compounded, which can adversely impact the quality of power being supplied to the customer, and can lead to equipment damage as described above.
Accordingly, it is desirable to provide an inexpensive and reliable switching device for grounded capacitors. Moreover, it is desirable to provide a capacitor switching device that accurately switches capacitors into a multiphase transmission system at an appropriate time relative to the voltage signal in the respective lines to decrease voltage transients and decrease the likelihood of pre-striking. In addition, it is also desirable to provide a switching device for capacitors that is easy to manufacture, install and maintain.